pagemap: Use pread to read pagemap entries

When reading pagemaps, we read it from specific position. To
do it, we called lseek, then read. Fortunetely, there's a
syscall that does both things in one call -- pread. Since
we don't need to keep pagemap's position for further reads,
it perfectly suits our needs.

This removes 75% of lseek calls when dumping basic container.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>